The state of the substance has characteristic qualities. The possibility or absence of the likelihood of maintaining shape and volume, the existence or absence of short-range and long-range order, etc. is a “state of aggregation."
When the aggregate form changes, drastic changes in density, entropy, free energy, and other physical properties can occur.
Substances can exist in three main states: gaseous, solid and liquid. But not everyone is able to transform into each of them. For some, only two or one state of aggregation is characteristic.
The gaseous form of a substance is characterized by particles that are weakly bound or not bound by interaction forces, the kinetic energy of motion of which is several times greater than the potential energy of their interaction. In view of this, the particles have freedom of movement and completely fill the container in which they are located, acquiring its shape.
The substance in liquid form has very mobile particles, between which there are small gaps. Its density exceeds the density in the gaseous state. Fluids are inherently fluid.
Solid bodies are characterized by stability of shape and a certain thermal motion of atoms caused by their vibration. Compared to the gaps between atoms, their vibration amplitude is negligible. For example, the state of aggregation of iron is solid. It has a stable shape. Loose but solid substances also belong to this group. Some people do not know the state of aggregation of sugar or salt. Answer: "Firm."
Such states as polymorphism (more than two polymorphic modifications) and plasma (fully or partially ionized gas) are also distinguished .
All simple substances or chemical compounds are inherent in one form of a liquid state and one in a gaseous state (with the exception of liquid crystals).
But the solid state of the same substance can have two or more modifications that differ in properties and internal structure. The existence of several forms of the solid state of a particular compound or simple material is called polymorphism.
The relative stability of a particular modification or phase is affected by environmental conditions: pressure and temperature. The aggregate state and polymorphic form of a substance depend on their indicators.
The process of transition from liquid to gaseous form is called vaporization. And if vaporization is carried out only from the surface of the liquid - this is called evaporation. The conversion of a substance from a gaseous state back to liquid is called liquefaction.
Sublimation (sublimation) refers to the conversion from solid to gaseous state, and the reverse process is called “desublimation”. Sublimation can be considered by the example of freezing of window panes in a room, frost falling onto the soil and other natural phenomena.
The process of converting a gaseous form into a liquid (liquefaction) or solid (desublimation) is defined as a single concept - vapor condensation. Based on this, such a state is considered as condensed.
The transition to the liquid form from solid is called melting, and the reverse transformation is called solidification, and under the influence of low temperatures - freezing. The implementation of the transformation of the modification of the solid form into another is called the "polymorphic transformation" (transition).
While the aggregate state of a substance changes or a polymorphic transformation is carried out, heat is released or absorbed. This happens, for example, during sublimation, vaporization, melting, evaporation, etc.
Therefore, heat is absorbed or released, conclusions can be drawn about the stability of the polymorphic form or state of aggregation of a substance under certain temperature conditions. The forms into which the transition is accompanied by heat absorption are more stable at high temperatures. And low temperatures are more suitable for those in which the transition is accompanied by heat. Evaporation and melting proceeds accompanied by absorption of heat, as a result of which the liquid state is much more stable than solid at high temperatures. And in an environment with a higher temperature, the gaseous form will be more stable than liquid.